1. Technical Field
The present invention relates to submarine communication networks, and more particularly, to submarine optical communication networks with reconfigurable branching units.
2. Description of the Related Art
Submarine optical networks for the backbone of global communication networks. They connect the telecommunication and data communication traffics among different continents. Together with the terrestrial networks, the submarine optical networks provide the pipeline for the users from different locations in the world to communicate with one another. Besides providing communication between continents, the submarine optical networks are also used to establish connection between islands or provide communication among coastal cities (instead of laying fiber underground on land and setting up terrestrial fiber links.)
Comparing with terrestrial optical networks, submarine networks usually have higher capacity. Because there are fewer resources (fewer fibers) in the submarine networks due to higher equipment and deployment cost, more traffic is aggregated into each fiber. This becomes more significant as the global traffic demand creases exponentially as a result of the growing Internet services. Submarine optical networks usually consist of main trunks that connect the trunk terminals, which are point-to-point cable links with in-line optical amplifiers to boost the signal power, and branch path that connect to other branch terminals. A branching unit (BU) is a network element that splits the signal between the main trunk and the branch path and vice versa. This allows the signals from different paths to share the same fiber, instead of installing dedicate fiber pairs for each link.
A drawback of using conventional BU's in a submarine network is that conventional BU's employ a pre-determined, fixed wavelength arrangement; and therefore no reconfiguration is possible during normal use. As the submarine network traffic becomes more dynamic, and Internet-based traffic becomes more dominant, the pre-determined, fixed wavelength assignment will not always be optimum. Upgrades or reconfigurations may be required after deployment, and upgrades or reconfigurations are very difficult to perform using conventional BU's since the equipment is under the sea. Another problem with upgrading or reconfiguring conventional BU's in a submarine network is that long periods of interruption to existing traffic will occur during any upgrades or reconfigurations, which may affect an extremely large amount of users across multiple continents.
One technique which attempts to reconfigure BU architectures involves changing the fixed wavelength optical filters in a BU to tunable filters. By changing the passband and stop band wavelengths, different amount of optical spectrum can be passed to different terminals according to the requirement. However, a major disadvantage of the tunable based BU is that tunable optical filters have fixed passband width; therefore the amount of spectrum for each path is also fixed and cannot be changed by tuning, even though the center of the passband can be shifted. This significantly reduces the reconfiguration capability. Tunable optical filters with the tunability at both the center frequency and the passband width have been proposed to increase the reconfiguration ability. However, even though the passband width can be set dynamically, the passband has to be contiguous, similar to all other optical filters. This also limits the reconfigurability of the spectrum usage among different paths.
Therefore, there is a need for the next generation of submarine network BU's to have reconfigurability in wavelength assignments, with readily reconfigurable BU's as a key enabling element. This reconfigurability will also help to maintain the submarine link at the optimal operation condition when the transmitted signal upgrades (such as increase the channel data rate or using more advanced modulation formats, which might require new channel spacing or mixed channel widths). Furthermore, due to the physical location and environment, the time and effort to repair damages in submarine networks is much greater than in the terrestrial network. Thus, there is a need for optical switches in the BU's to have a latching feature, which means that the switches will maintain their switching setting even after the power is turned off or cut. This latching feature also reduces the power consumption in the BU, which is highly desirable.